1. Field of the Invention
The present invention relates generally to the formation of patterns for resists and other protective materials employed on semiconductor devices. More specifically, the present invention relates to a method for forming patterns in a resist or other protective material by traversing an energy beam thereover instead of exposing the material to radiant energy through a mask.
2. State of the Art
The manufacture of semiconductor devices routinely involves multiple masking steps. For example, resist material, also commonly termed xe2x80x9cphoto resistxe2x80x9d material due to its light sensitivity, is deposited on a semiconductor substrate and patterned by exposing a portion of the resist material to ultraviolet light through a mask which defines a positive or negative resist pattern. In the case of a positive resist pattern, the exposed resist material is chemically modified and/or degraded by the ultraviolet light such that the exposed resist material portions may be removed by conventional developing techniques as known in the art. In the case of a negative resist pattern, the exposed resist material is chemically modified to harden or set by exposure to the ultraviolet light such that the exposed resist material portions are not degraded by conventional developing techniques, but instead the unexposed resist material may be removed by developing, resulting in a resist pattern on the semiconductor matching the mask pattern. Suitable additional procedures, such as deposition, etching, or the like, are carried out after the resist is patterned to form desired features of a semiconductor device. The resist material is then removed, leaving the desired semiconductor features, and manufacturing continues. The masking process may be repeated as desired with differently patterned masks in conjunction with associated material deposition and removal steps until all of the desired features of the semiconductor have been formed.
FIGS. 1 and 2 illustrate an exemplary process of creating a resist pattern on a semiconductor device. FIG. 1 illustrates a semiconductor device 100 covered by a layer of resist material 110. A mask 120 having both radiation-transparent portions 122 and radiation-opaque portions 124 is aligned over the resist material 110. Electromagnetic radiation 130 (such as ultraviolet light, x-rays or visible light) is directed perpendicularly toward the mask 120 and passes through the transparent portions 122 of the mask 120 to be absorbed by the resist material 110. Those portions of the electromagnetic radiation 130 meeting the opaque portions 124 of the mask 120 are either reflected or absorbed by the material of mask 120. The resist material 110 exposed to the electromagnetic radiation 130 through transparent portions 122 of mask 120 undergoes a chemical change, as previously discussed above (depending upon whether the resist is of the positive or negative type), to define a pattern including resist features 140 as illustrated in FIG. 2.
The miniaturization of semiconductor devices requires the definition of higher resolution resist patterns having smaller dimensional tolerances and variances. With increasing frequency, the masking processes currently available are being replaced with processes which provide superior resolution and miniaturization of the resist patterns. For example, a dual mask process is disclosed in U.S. Pat. No. 5,851,734 to Pierrat, wherein the use of two masks during a masking process produces better resolution of a desired resist feature than can be accomplished with one mask. In Pierrat, a first resist portion is exposed by a first mask which is used to define one edge of a desired resist feature. A second mask defines the second edge of the desired resist feature. In a positive mask, only that portion of the resist material exposed by both the first and the second mask remains following etching. Similarly, in negative masking, that portion of the resist material which is exposed by both the first and the second mask is removed from the resist layer. This masking technique provides a method whereby smaller resist features can be formed.
In addition to the use of masking processes as described above to create resist patterns for definition of semiconductor device features, masking processes may also be employed to form a layer of protective material over portions of die locations on a semiconductor wafer. For example, a protective material may be deposited in a selected pattern over wire bonds or conductive traces to protect such features from damage or unwanted coating during a particular stage of processing, handling or testing. A portion of a protective material layer covering a semiconductor wafer is exposed, for example, to ultraviolet light through a mask defining the desired protective pattern. The exposed or unexposed portion of the protective material (depending upon its chemical makeup) is then developed and washed from the semiconductor wafer, leaving a pattern of material adhered to the wafer protecting selected locations.
In addition to the trend toward increased miniaturization, another significant consideration in semiconductor device fabrication is process speed. Significant economic advantages may be realized when the rate of production of semiconductor devices fabricated during a given time period is increased. Conventional masking processes require a series of steps including the application of a resist material to a semiconductor substrate such as a wafer, alignment of a mask over the active surface of the wafer, exposure of the resist material to appropriate wavelength radiation through at least one mask to define a desired pattern on the resist, developing of the resist material and rinsing of the excess resist material from the semiconductor wafer. Elimination of a single step in the resist patterning sequence would save substantial time and boost production rates. If the masking step were to be eliminated, not only process time but also the considerable expense of the mask itself would be saved. If the development step were to be eliminated, even further production efficiencies would be realized.
In addition to the cost and relatively slow speed involved in mask placement and alignment, conventional masking of resist material must be effectuated on an entire semiconductor substrate including hundreds or thousands of semiconductor die locations separated by so-called xe2x80x9cstreetsxe2x80x9d before the dice are singulated therealong. Thus, alignment must be accurate across an entire substrate such as a wafer, which is proving to be increasingly difficult as feature dimensions shrink, wafers become larger as technology advances (the next generation wafers in development are 30 cm in diameter) and non-planarity of wafer surfaces across the larger wafers becomes more significant to the fabrication process.
The use of lasers in selected aspects of semiconductor fabrication is becoming more common. One use of lasers in semiconductor fabrication is for the marking of a semiconductor package with part numbers, serial numbers, or other information. Laser marking techniques are desirable because of the enhanced efficiency, accuracy and speed of laser marking. The beams of such marking lasers are controlled, manipulated, and triggered to mark the semiconductor only in a specified or programed pattern of indicia. U.S. Pat. No. 5,838,361 to Corbett and U.S. Pat. No. 5,937,270 to Canella each describe exemplary apparatus, methods and techniques used in laser marking of semiconductor chips. The disclosure of each of the Corbett and Canella patents is hereby incorporated herein by reference.
Lasers are also employed in stereolithographic processes used to form layered structures by selectively exposing portions of a photopolymer film to ultraviolet wavelength laser radiation, as disclosed in more detail in the aforementioned U.S. patent applications Ser. Nos. 09/259,142 and 09/259,143. Such processes have been specifically adapted and improved for use in certain aspects of semiconductor fabrication as disclosed in the patent application.
A wide variety of photoresist materials are well known in the art. A resin polymer-based negative photoresist sensitive to near-UV radiation is described in xe2x80x9cEPON SU-8: A LOW-COST NEGATIVE RESIST FOR MEMS,xe2x80x9d Lorenz et al., and xe2x80x9cHigh-aspect-ratio, ultrathick, negative-tone near-UV photoresist and its applications for MEMS,xe2x80x9d Lorenz et al., Sensors and Actuators A 64, pp. 33-39 (1998), the disclosures of each of which are incorporated herein by reference.
It would be advantageous to provide a high speed, accurate, maskless resist patterning process wherein a laser is used to define a pattern in a resist or other protective material on a semiconductor wafer. It would also be advantageous to employ such a process in a manner applicable to partial wafers or even individual semiconductor dice, as desired.
According to the present invention, a method for forming resist or other protective patterns on a substrate is disclosed using exposure of a layer of heat- or selected wavelength-sensitive protective material to a focused, controlled laser or other energy beam comprising electromagnetic radiation traversed over the material layer in a selected path such that portions of the material layer are cured to define a protective pattern over the substrate. As an alternative to traditional masking techniques, the present invention may be used to form patterns of conventional configurations on substrates, such as semiconductor wafers, partial wafers or singulated devices, with at least as great precision and greater repeatability and speed, than is achievable using conventional masking techniques. Furthermore, the present invention may be used to create protective formations over wires and other conductors during semiconductor fabrication without the use of masking techniques.
The method of the present invention involves covering at least a portion of a substrate with a layer of a heat-sensitive or selected wavelength-sensitive protective material, exposing selected portions of the material to a beam of electromagnetic radiation such as a laser beam to completely cure those portions of the material and washing away the uncured portions of the material remaining on the substrate. A precuring step may also be employed to partially cure the entire material layer before exposure of selected portions thereof to the electromagnetic radiation beam. The cured material remaining after developing and/or washing defines the desired protective pattern on the substrate.
In one exemplary embodiment, a layer of EPON Resin SU-8 formulated in solution as a photoresist is deposited on a substrate to a desired thickness and partially cured to a B-stage. Portions of the partially cured resin material are subjected to a traversing laser beam having an appropriate wavelength and energy density sufficient to initiate the cure of the B-stage resin material to a C-stage. Those portions of the partially cured resin material not exposed to the laser beam are removed from the substrate, leaving behind a protective pattern consisting of those portions of the resin material cured to a C-stage.